1. Field of the Invention
The invention is concerned with methods for making an organic electronic device. The present invention also is concerned with electronic devices preparable by the present methods. The invention also is concerned with hole transport materials for use in electronic devices and to methods for making the same.
2. Related Technology
Such organic devices include organic light-emitting diodes (OLEDs). One or more of the layers in the device typically will comprise a polymer. Further, such devices typically comprise one or more semiconductive polymer layers located between electrodes. Semiconductive polymers are characterised by partial or substantial pi-conjugation in the backbone or side chains.
Semiconductive polymers are now frequently used in a number of optical devices such as in polymeric light emitting devices (“PLEDs”) as disclosed in WO 90/13148; field effect transistors (“FETs”); photovoltaic devices as disclosed in WO 96/16449; and photodetectors as disclosed in U.S. Pat. No. 5,523,555.
A typical LED comprises a substrate, on which is supported an anode, a cathode, and an organic electroluminescent layer located between the anode and cathode and comprising at least one electroluminescent material. In operation, holes are injected into the device through the anode and electrons are injected into the device through the cathode. The holes and electrons combine in the organic electroluminescent layer to form an exciton, which then undergoes radiative decay to give light. Other layers may be present in the LED. For example a layer of organic hole injection material such as poly(ethylene dioxythiophene) (PEDT) doped with a charge balancing dopant may be provided between the anode and the organic electroluminescent layer to assist injection of holes from the anode to the organic electroluminescent layer. The charge balancing dopant may be acidic. The charge balancing dopant may be a polyanion. Preferably the charge balancing dopant comprises a sulfonate, such as poly(styrene sulfonate) (PSS).
Further, a layer of an organic hole transport material may be provided between the anode (or the hole injection layer where present) and the organic electroluminescent layer to assist transport of holes to the organic electroluminescent layer.
Generally, it is desired that the polymer or polymers used in the afore-mentioned organic devices are soluble in common organic solvents to facilitate their deposition during device manufacture. Many such polymers are known. One of the key advantages of this solubility is that a polymer layer can be fabricated by solution processing, for example by spin-casting, ink-jet printing, screen-printing, dip-coating etc. Examples of such polymers are disclosed in, for example, Adv. Mater. 2000 12(23) 1737-1750 and include polymers with at least partially conjugated backbones formed from aromatic or heteroaromatic units such as fluorenes, indenofluorenes, phenylenes, arylene vinylenes, thiophenes, azoles, quinoxalines, benzothiadiazoles, oxadiazoles, thiophenes, and arylamines with solubilising groups, and polymers with non-conjugated backbones such as poly(vinyl carbazole). Polyarylenes such as polyfluorenes have good film forming properties and may be readily formed by Suzuki or Yamamoto polymerisation which enables a high degree of control over the regioregularity of the resultant polymer.
In certain devices, it can be desirable to cast multiple layers, i.e., laminates, of different materials (typically polymers) on a single substrate surface. For example, this could be to achieve optimisation of separate functions, for example electron or hole charge transport, luminescence control, photon-confinement, exciton-confinement, photo-induced charge generation, and charge blocking or storage.
In this regard, it can be useful to be able to fabricate multilayers of materials (such as polymers) to control the electrical and optical properties, for example, across the device. This can be useful for optimum device performance. Optimum device performance can be achieved, for example, by careful design of the electron and hole transport level offset, of the optical refractive index mismatch, and of the energy gap mismatch across the interface. Such heterostructures can, for example, facilitate the injection of one carrier but block the extraction of the opposite carrier and/or prevent exciton diffusion to the quenching interface. Thereby, such heterostructures can provide useful carrier and photon confinement effects.
However, preparation of polymer laminates generally is not trivial. In particular, the solubility of initially cast or deposited layers in the solvents used for succeeding layers can be problematic. This is because solution deposition of the subsequent layer can dissolve and destroy the integrity of the previous layer.
One option for overcoming this problem is to work with precursor polymer systems. Precursor systems of PPV (polyphenylene vinylene) and PTV (polythienylene vinylene) are known in this art.
Layers of semiconducting polymers may be formed by depositing a soluble polymeric precursor which is then chemically converted to an insoluble, electroluminescent form. For example, WO 94/03030 discloses a method wherein insoluble, electroluminescent poly(phenylene vinylene) is formed from a soluble precursor and further layers are then deposited from solution onto this insoluble layer.
However, it is clearly undesirable to restrict the polymer in a polymer device to that class of polymers that may be formed from insoluble precursor polymers. Furthermore, the chemical conversion process required for precursor polymers involves extreme processing conditions and reactive by-products that may harm the performance of the prior layers in the finished device.
A number of publications disclose devices where two layers are solution processed during device manufacture such that the solvent use for the second layer does not dissolve the first layer.
One approach is to form the first layer and then to crosslink the first layer to render it insoluble so that the second layer then can be formed.
WO96/20253 generally describes a luminescent film-forming solvent processable polymer which contains crosslinking. It is stated that because the thin films resist dissolution in common solvents this enables deposition of further layers, thereby facilitating device manufacture. The use of azide groups attached to the polymer main chain is mentioned as an example of thermal crosslinking.
U.S. Pat. No. 6,107,452 discloses a method of forming a multilayer device wherein fluorene containing oligomers comprising terminal vinyl groups are deposited from solution and cross-linked to form insoluble polymers onto which additional layers may be deposited.
Similarly, Kim et al, Synthetic Metals 122 (2001), 363-368 discloses polymers comprising triarylamine groups and ethynyl groups which may be cross-linked following deposition of the polymer.
Problems exist with these crosslinking methods since the device must be subjected to crosslinking conditions e.g. heating after the deposition of a layer. This can have detrimental effects on the already deposited layer. Crosslinking methods also can result in side products, which can contaminate the film. Further, disadvantageous side radical reactions can occur. These side radical reactions result in less than the maximum degree of crosslinking being obtained and the functionality of the polymer being affected.
WO 2004/023573 is concerned with a method of forming an optical device comprising the steps of providing a substrate comprising a first electrode capable of injecting or accepting charge carriers of a first type; forming over the first electrode a first layer that is at least partially insoluble in a solvent by depositing a first semiconducting material that is free of cross-linkable vinyl or ethynyl groups and is, at the time of deposition, soluble in the solvent; forming a second layer in contact with the first layer and comprising a second semiconducting material by depositing a second semiconducting material from a solution in the solvent; and forming over the second layer a second electrode capable of injecting or accepting charge carriers of a second type wherein the first layer is rendered at least partially insoluble by one or more of heat, vacuum and ambient drying treatment following deposition of the first semiconducting material.
JP 2003-217863 takes a different approach. In particular, this disclosure teaches the presence of a compound in a solution deposited layer, where the compound can render the layer insoluble upon heating. In one example, a hole transport layer of F8 doped with an electron acceptor of an antimony hexachloride salt of tri(bromophenyl) amine is used. The layer is deposited from tetrahydrofuran and insolubilised by heating at 100° C. for 20 hours. An emissive layer of F8 then is deposited from xylene solution. Doping of the F8 polymer alters its charge transporting properties, providing it with hole transporting functionality. However if a soluble material that possesses suitable hole transporting functionality is treated with a doping agent, then this will result in an undesirable alteration of that functionality.
A further option for overcoming this problem is to use polymers materials that differ widely in their solubility behaviour so that a different solvent (in which the first layer is not soluble) can be used to deposit the second layer. Again, this option severely restricts the classes of materials that can be used in a multilayered stack. This is because most conjugated polymer systems are characterised by solubility in the same set of hydrocarbon solvents (such as xylenes and other substituted benzenes and tetrahydrofuran, and halogenated solvents).
For example, the use of a polymer that is soluble in a hydrocarbon solvent in conjunction with a polymer that is soluble in water or in an acetate solvent can allow the preparation of a limited bilayer or trilayer stack. An important example in this respect is the deposition of a conjugated polymer from an aromatic hydrocarbon solvent over a first-formed conductive PEDT:PSS film that is not soluble in the aromatic hydrocarbon solvent.
US 2002/096995 discloses the following multilayer structure in Example 1:                ITO;        Poly(ethylene dioxythiophene)/poly(styrene sulfonate) “PEDT/PSS”;        emissive layer of Ir(ppy)3/“PVK” by spin coating from 1,2-dichloroethane;        electron transport layer of 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene “TPBI”/polyvinylbutyryl binder by spin coating from 1-butanol.        
JP 2002-050482 is concerned generally with providing a high brightness, efficient, organic LED. Generally, this disclosure is concerned with an organic light-emitting device with two different luminescent layers. The first luminescent layer contains an ortho-metallated complex and has a first emission spectrum and the second luminescent layer contains a macromolecule which has a different emission spectrum from the first luminescent layer. This arrangement is said to lead to an efficient device with high brightness. The following multilayer structure is disclosed in Example 1:                ITO;        poly(ethylene dioxythiophene) “PEDT”;        “First luminous layer” of Ir(ppy)3/poly(N-vinylcarbazole)“PVK” deposited by spin coating from 1,2-dichloroethane;        “Second luminous layer” of a blend of poly(9,9′ n-octyl fluorene) “F8”, F8-amine copolymer and a diazole deposited by spin-coating from xylene solution.        
JP 2003-077673 also is concerned with achieving high brightness and efficiency in organic electroluminescent devices. In one example in JP 2003-077673, a device has a hole transport layer, an emissive layer and an electron transport layer. The device is formed by solution processing of two of the layers wherein the solvent used for the emissive layer does not dissolve the hole transport layer and/or the solvent used for the emissive layer does not dissolve the electron transport layer. In one example, the device structure is as follows:                hole transport layer of PVK formed by spin-coating from 1,2-dichloroethane;        emissive layer of F8/Ir(ppy)3 formed by spin-coating from xylene;        electron transport layer Alq formed by evaporation.        
JP 2002-319488 is concerned with avoiding problems associated with using vacuum over evaporation during device manufacture. In Example 1, the following multilayer structure is used:                hole transport layer of PVK formed by spincoating from 1,2-dichloroethane;        emissive layer of polystyrene/polyvinyl biphenyl “PBD”/Ir(ppy)3 formed by spin-coating from cyclohexane.        
In Example 3, the following multilayer structure is used:                hole transport layer of PVK formed by spin-coating from 1,2-dichloroethane;        emissive layer of polyvinyl biphenyl/“OXD-1”/Ir(ppy)3 formed by spin-coating from xylene; “OXD-1” is defined in JP 2002-319488;        electron transport layer of polystyrene/PBD deposited from cyclohexane.        
JP 2003-007475 again is concerned with providing an efficient electroluminescent element with high brightness. In particular, this disclosure is concerned with lowering the voltage needed to drive the device. The following multilayer structure is disclosed:                hole transport layer of dibutyl fluorene deposited from 1,2-dichloroethane;        emissive layer of “polyvinyl biphenyl”/carbazole biphenyl “CBP”/Ir(ppy)3 formed by spin-coating from xylene;        electron transport layer of OXD-1 as defined in JP 2003-007475 formed by evaporation.        
In these disclosures, the devices are designed within the confines of selecting materials with the desirable solubility behaviour so that laminates may be made. It will be appreciated that limitations on the materials that are useable in the laminate mean that many concepts of device structure cannot be investigated or implemented. As such, the further development of device architecture may become heavily impeded.
J. Am. Chem. Soc. 1996, 118, 7416-7417 discloses the use of BDOH-PF in a single layer LED and LEC. BDOH-PF is soluble in THF and toluene and other common organic solvents. An LED is fabricated by sandwiching a thin film of BDOH-PF (as the emitter) between an ITO-coated glass anode and a vacuum evaporated cathode. An LEC is fabricated using a blend of BDOH-PF with lithium triflate. A thin film of the blend was sandwiched between and ITO-coated glass anode and a vacuum evaporated aluminium film.
WO 01/47043 is concerned with a method for forming a transistor comprising: depositing a first material from solution in a first solvent; and subsequently whilst the first material remains soluble in the first solvent, forming a second layer of the transistor by depositing over the first material the second material from solution in a second solvent in which the first material is substantially insoluble. According to one preferred embodiment described on page 4 of WO 01/47043 one of the first and second solvents is a polar solvent and the other of the first and second solvents is a non-polar solvent. As such, one of the first and second layers may be a non-polar polymer layer that is soluble in a non-polar solvent and the other of the first and second layers may be a polar polymer layer that is soluble in a polar solvent. There is no mention of a hole transport material or a hole transport layer. As described above, the invention that is the subject of WO 01/47043 works within the confines of selecting materials with the desirable solubility behaviour, thus, similar limitations exist on the materials that are useable in the invention according to WO 01/47043.
Chem. Mater., 2004, 16, 708-716 discloses two conjugated polyelectrolytes (P2, P4), which are soluble in polar solvents. Further, J. Am. Chem. Soc., 2004, 126 (31), 9845-9853 discloses the quarternised ammonium polyelectrolyte derivatives of a series of aminoalkyl-substituted polyfluorene copolymers with benzothiadiazole, which were synthesised by the Suzuki coupling reaction. The quarternised polymers are soluble in DMSO, methanol, and DMF. It is said that the solubility of these quarternised polymers in alcohol offers an opportunity of fabricating multilayer polymer LEDs by spin coating from such solvents, since most of the electroluminescent polymers and carrier transporting materials are not soluble in the alcohol.
Reference may also be made to U.S. Pat. No. 5,900,327, which discloses the use of fluorenes and polyfluorenes having one or two polar substituents, for use as luminescent materials in organic light-emitting devices.
WO 99/48160 is concerned with an electroluminescent device comprising a first charge carrier injecting layer for injecting positive charge carriers; a second charge carrier injecting layer for injecting negative charge carriers; and a light-emissive layer located between the charge carrier injecting layers and comprising a mixture of a first component for accepting positive charge carriers from the first charge carrier injecting layer; a second component for accepting negative charge carriers from the second charge carrier injecting layer; and a third, organic light-emissive component for generating light as a result of combination of charge carriers from the first and second components. At least one of the first, second and third components forms a type II semiconductor interface with another of the first, second and third components.
In view of the above it will be understood that there still remains a need to provide further methods for fabricating multilayer organic (typically polymer) electronic devices.